Java help for an Excel person, please.

What I'm trying to do is to attach a macro to a button (which I can do) that changes the contents of a range of cells based on whether or not they contain numbers.  Like ticking off a to do list, in a sense.  The cells in range D11 through W15 (5 rows of 20) are each numbered 1 through 100.  Upon clicking the button, I would like to make a script that checks the first cell is still a number and if so, change the cell value to "X" instead.

I have no experience with java but plenty with VBA and am now in a new job where I can only use google sheets instead of excel.  I can imagine ways to do this with a for next loop or a while wend loop but I don't even know the syntax for java, let alone what commands and functions it has to do this.  Thanks, experts!